The term "believe" is wrong. Iit's not a matter of belief. Aliens are more a theoretical nature. We will possibly never know it. Look up the fermi paradox.
Aliens could possbile be too alien to communicate with us or they simply fear other beings and avoid them at all cost. Or they already exist togehter with us, but they are so perfect under us, it's impossible to detect the [or mabye not?]
And they exist even more theories. Good stuff for any Sci-Fi medium.